The observable universe

i'm reading through a book called "Discovering The Universe" and in the chapter about cosmology the familiar idea of the observable universe is discussed.

It speaks of the radius of the cosmic particle horizon being ~15 billion light years, which makes perfect sense as light from further away has not had time to reach us yet.

But I have a question. Have we ever seen, or are we actively looking for new stars in the night sky? Am I right in thinking as time goes on, it's possible that light from a never before seen star will reach earth and will effectively 'appear' in the sky.

The size of the observable universe is increasing slightly over time so yes, new stars will move into it, BUT ... light from a single star at that distance will be too weak to detect. Google the "Hubble Deep Field" for a discussion of how GALAXIES at fairly close to that distance are detectable, but only with some effort.

I don't think we will be seeing any light that is older than the universe. The CMB is the effective limit on how far we can see into the universe in EM wavelengths. It is not quite as old as the universe itself. To see any further we would require neutrino or gravity wave detectors. These are a work still in progress.

I don't think we will be seeing any light that is older than the universe. The CMB is the effective limit on how far we can see into the universe in EM wavelengths. It is not quite as old as the universe itself. To see any further we would require neutrino or gravity wave detectors. These are a work still in progress.

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I assume you mean that we can't see light that is older than the universe as we know it?
I'm not trying to post personal views here, but it is tempting to believe that the universe doesn't expand faster than light, so that what we can observe (if we could observe the most distant object) in fact is the limit of the scope of the universe (?).

The 90 billion light years thing is a source of confusion. It is true the observable universe is believed to be this large NOW, but, we cannot and will never see the universe as it is NOW. We see the universe at the age it was when photons we detect were emitted. For example, the CMB, the most ancient photon source possible to detect, was a mere ~40 million light years distant when the photons we now detect were emitted. Due to expansion, it took those photon 13.7 billion years to reach us. In the mean time, the CMB has receded to 90 billion light years, also due to expansion.

As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55.

Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

Code (Text):

O B A C
* * * *

* * * *
O B A C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, [itex]t_e[/itex]. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, [itex]t[/itex]. Notice that space has "expanded" between times [itex]t_e[/itex] and [itex]t[/itex].

Suppose that at time [itex]t_e[/itex]: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time [itex]t[/itex], galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time [itex]t_e[/itex] is greater than c, the pulse fired towards us has actually moved away from us between times [itex]t_e[/itex] and [itex]t[/itex].

Now, suppose that the distance from us to galaxy B at time [itex]t[/itex] is the same as the distance to galaxy C at time [itex]t_e[/itex]. Even though the distances are the same, the recession speed of B at time [itex]t[/itex] is less than than the recession speed of C at time [itex]t_e[/itex] because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times [itex]t_e[/itex] and [itex]t[/itex].

Since A's recession speed at time [itex]t_e[/itex] is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time [itex]t_e[/itex] also is greater than c. If, however, the Hubble constant decreases enough between times [itex]t_e[/itex] and [itex]t[/itex], then B's recession speed at time [itex]t[/itex] can be less than c. If this is the case, then at time [itex]t[/itex] (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times [itex]t_e[/itex] and [itex]t[/itex].

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time [itex]t_e[/itex], galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time [itex]t[/itex], galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

The size of the observable universe is increasing slightly over time so yes, new stars will move into it, BUT ... light from a single star at that distance will be too weak to detect. Google the "Hubble Deep Field" for a discussion of how GALAXIES at fairly close to that distance are detectable, but only with some effort.

Click to expand...

Light would be Red-Shifted if the source is moving away. The universe is expanding and a new star just forming at the edge of the universe would probably be moving away also.

Light would be Red-Shifted if the source is moving away. The universe is expanding and a new star just forming at the edge of the universe would probably be moving away also.

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Yes, but the fact that is moving away doesn't mean its light wouldn't reach us, it just means that as you say it would be red-shifted. It also would be incredibly faint, both of which combine to mean that it would not likely be detectable. "Not detectable" however, is not the same as "does not exist".

But, such a star would not be 'new' in the sense of expansion overtaking a hitherto unobservable region of space. Since all photons originate inside the CMB, the region of the universe containing that star has always been observable. Remember too, that all events we observe in the remote universe are also time dilated. So the 'birth' of an object at high redshift would appear to be in slow motion as observed from earth. This why distant SN1a take longer to peak in luminosity than low z supernova.

We won't detect any new stars or galaxies at that distance not only because the stars would be too faint, but because we'd be waiting for a very long time to detect much of anything. For example, if an undetected galaxy is relatively close to the edge of the visible universe--let's say 1 million LY--we wouldn't be able to detect it for a million years. The timescale is simply so great that we don't look for changes in things like that.